Grain and other particulate material falling uncontrolled at high velocities causes: (1) damage to the grain and other particulate material and (2) creates dust. The damage occurs both during fall in a downwardly inclined conduit, as well as upon impact after discharge from the conduit. The damage is apparently caused by particle-to-conduit abrasion and particle-to-particle impact, and causes fines and dust.
There exists a particular fall velocity above which the affect on the quality and value of the grain or other particulate material becomes unacceptable. This "damaging velocity" can vary with the type and density of the grain or other particulate material, its dryness, and the type of conduit, but in many cases it is in the vicinity of about 1600 feet per minute. If the grain or other particulate material velocity exceeds the threshold of damage for that stream, grain or other particulate material quality is affected to an unacceptable degree.
The velocity increase is of course most rapid in a vertical conduit, but even in a slanting conduit, velocities of several hundred feet per minute are attained in a few feet. A free-falling stream tends to reach a terminal velocity because the air currents cause turbulence within an unconfined stream. Grain or other particulate material flowing in conduits can reach velocities well above such terminal velocities upon discharge. In a long conduit, the velocity of the stream can exceed even two thousand feet per minute. The damaging velocity is apparently not highly dependent on the angulation of the conduit; that is, the damaging velocity is roughly the same for a given stream whether it is falling in a vertical conduit or an angulated conduit.
For a given type of grain or other particulate material, a velocity of 1600 feet per minute is the approximate damaging velocity in many cases. The same phenomenon of unreasonable damage to the material as the result of high conduit velocities also arises with materials other than grain, although it is most important in respect to grain because of the relative frangibility and layered structure of grain kernels.
To prevent a falling particulate stream of material from exceeding damaging velocity, the most straightforward approach is to limit the distance of the drop or to angulate the conduit so that the velocity increase is less. However, it is difficult to avoid the need for a substantial elevation change in many instances, as for example in loading grain or other particulate material into the hold of a ship.
In some situations, a flow restriction in the path of the falling material slows the material velocity. So called "dead boxes" are known for that purpose and comprise a narrow fixed throat section which can be mounted directly in the conduit. However, dead boxes are effective primarily in those situations where the grain or other particulate material flow rate (i.e., bushels per hour) is constant or within a relatively small range so that there are no "surges." That is not usually the case. In the typical situation, surges--abrupt increases in flow rate, followed by decreases--occur repeatedly. Surging occurs, for example, when a conveyor bucket empties into the upper end of the spout and a period of lower flow may follow, until another bucket refills the chute. Non-uniform moisture content in the grain or other particulate material is another cause of surges.
When the flow is uneven, the use of a fixed restriction to retard flow is of little effect. In periods of low flow rate, the small stream passes almost unrestrained through the throat opening; and at periods of heavy flow, a "head" of grain or other particulate material builds up rapidly above the throat. This can cause bridging or clogging which can choke off flow completely.
In addition to the problem of damage to the grain or particulate material due to high falling velocities, air entrained within the falling stream is a significant problem due to the creation of dust. Ambient dust as a result of a falling stream of particulate material typically requires workers to wear masks or other protective gear. Furthermore, the work environment is clouded by the dust and workers have difficulty seeing for secure footing and evaluation of the level of fill of the vessel receiving the grain or particulate material.
One known method and apparatus for reducing the dust and fall velocity of grain or other particulate material in inclined conduits, vertical as well as angulated to vertical, by which the velocity can be prevented from exceeding the damaging value even under widely varying flow rates, is disclosed in U.S. Pat. No. 4,342,383, hereby incorporated by reference in its entirety.
The apparatus of the invention of the '383 patent comprises a variable accumulator and gate which is responsive to the rate of flow of the stream, and which reduces its velocity without causing clogging or bridging. This is achieved by the provision of a conical or slanting variable throat having an outlet opening area substantially smaller than the cross section of the incoming stream and which establishes an accumulation of grain or other particulate material above the gate or opening. The accumulation rises, and its weight increases, as incoming flow rate increases. The weight of this accumulation is sensed and the area of the opening is increased as the weight increases which provides for an increased rate of release of grain or other particulate material through the gate. Under normal conditions the gate slows the velocity nearly to zero at the point of accumulation, but since the accumulation is discharged more rapidly as it builds, the danger of bridging or clogging is reduced.
An array of downwardly and inwardly sloping blades is supported by the body of the accumulator in the '383 patent. The blades have lower ends which define an opening between them, the opening having an area that at its maximum is substantially smaller than the area of the conduit. The overlapping blades are angulated inwardly so that they deflect the grain particles centrally as they fall. The inward deflection of the particles toward the smaller area of the throat causes a mass of grain or other particulate material particles to accumulate above the blades and over the opening in an accumulation chamber within the body.
In that invention, a variable biasing means acts on the blades to urge them inwardly and the biasing means is responsive to the weight of the accumulated mass of the particles to provide a larger opening as the weight increases, thereby to release particles more rapidly from the accumulation space above the blades and to reduce the area of the opening as the weight of particles decreases. The blade biasing means of the '383 patent comprises a torsion spring which acts on each blade adjacent the lower end thereof.
However, the adjustability of the torsion spring biasing mechanism disclosed in the '383 patent in some instances is inadequate. In an accumulator or regulating device, such as that disclosed in the '383 patent, it is important to keep the biasing force of the blades on the grain or other particulate material flow without applying excessive force to cause the grain or other particulate material to back-up and the accumulation mass above the gate or opening of the blades to grow too large and become clogged or jammed. A single accumulator or regulator system is often used for a variety of grain or other particulate material types, flow diameters, flow speeds, drop heights or quantities. Variable kinds of grains include grains of varying moisture content and varying weight. Such a variety of grains and other materials produces variable input forces and requires a variable biasing force to most efficiently and accurately regulate the flow preferably without constant attention by an operator. Furthermore, the regulator or accumulator should be highly sensitive to these variable flow parameters without damaging the grain or other material flowing therethrough or create an excess of dust or the like.